Efficient digital data utilization of limited frequency band channels ordinarily used for telephone voice transmission is accomplished by multilevel high speed signals, making possible, for example, transmission at 9600 bits per second. Low error rates, however, can only be achieved by effective compensation of signal degradation otherwise highly destructive to data signals produced by such transmission channels, although this degradation is widely tolerated since it is innocuous to voice transmission. Linear distortion due to variations in attenuation and delay imparted to different frequency components produces the dispersion effect commonly known as intersymbol interference. Transversal filters are generally used as an effective means to compensate for intersymbol interference.
In addition to linear distortion, there are also carrier-phase perturbation effects which are harmful to digital data signals. A primary source for such effects is the frequency translation oscillator of a frequency division multiplex (FDM) channel bank. The low-level, but omnipresent, power supply ripple in these oscillators contributes phase-jitter to the digital data signal at the fundamental and other low order harmonic frequencies of the AC power frequency.
In U.S. Pat. No. 3,878,468 issued to D. D. Falconer et al. on Apr. 15, 1977, an arrangement utilizing a phase-jitter compensator connected to the output of an equalizer is disclosed for providing jitter-free passband equalization of data signals. One disadvantage of this arrangement is that the linear channel distortion is compensated initially before phase-jitter is controlled which will distort any phase-jitter which may be present, i.e., the equalizer precedes the jitter control circuitry. Full cancellation of phase-jitter is possible by this prior art arrangement only if the phase-jitter occurs in the transmission path prior to the signal being subjected to linear channel distortion. This assumption is valid for some channels, but should the distortion effects occur in the reverse order the equalizer actually aggravates the phase-jitter as previously mentioned thereby frustrating effective phase compensation. The delay experienced by the correction signals in the Falconer receiver renders inoperable a reversal by changing the order of the equalizer and phase-jitter compensation, owing to potential instability problems.